[Bug] issue_repo_index not set for migrated repos

[6543]

gitea version: v1.15.0-rc1 or newer

introduced with #15599

reproduce:

1. migrate any repo with issues/pulls
2. try to create a new pull or issue

-> newIssue: Error 1062: Duplicate entry '424-1' for key 'UQE_tmp_recreate__issue_repo_index'